Mantra sidebar widths (2 posts)

I'm using Mantra 1.9.7 theme for WordPress. I'd like to take advantage of the 3 column layout (1 main and 2 sidebars), but I need the sidebars to be different widths? What would be the easiest way to achieve that? Fixed width is fine ... something like 100 pixels for the first and then 300 for the second.